Traditional navigation systems for manned vessels, such as inertial navigation systems, Doppler-aided inertial navigation systems, GPS-aided inertial navigation systems, etc., are highly accurate but relatively large, heavy, expensive, and power-hungry. Conversely, a navigation system for an unmanned underwater vehicle (UUV) is preferably small in size, lightweight and inexpensive, and should consume much less power than a traditional inertial navigation system in order to facilitate long duration, autonomous missions.
For example, the navigational load may be apportioned between a more accurate navigation system, to be used during a small portion of the UUV mission, such as a search phase, and a less accurate navigation system, to be used during the remaining portion of the UUV mission, such as the transit phase(s). Of course, the UUV navigation system used during the transit phase must satisfy particular navigational accuracy requirements, such as, for example, 10 nautical miles (nm) (1−σ) in both latitude and longitude.
While inexpensive sensors have been produced which can measure either latitude or longitude to an accuracy of about 10 nm, an inexpensive, compact, light weight, low power navigation system has yet to be developed which can estimate both latitude and longitude to the same accuracy, i.e., about 10 nm.
For example, in recent years wildlife scientists have attached inexpensive, miniature archival light sensors to fish in order to record their movements in the open ocean. These sensors typically record underwater light level, depth and temperature data. Contemporary sensors can sense light at approximately 300 meter depths and offer up to 16 MB of data storage. Latitude and longitude can be determined from the light measurements, depth measurements and an on-board clock. Accuracies depend on total daily sensor motion, depth, time of the year, and cloud cover patterns but can be, at the one sigma level, approximately 10 nm (about 20 km) in longitude and 130 nm (about 260 km) in latitude. These light sensors are typically better at measuring longitude than latitude, since the estimation of local noon (i.e., longitude) is more accurate than the estimation of the length of the day (i.e., latitude).
Inexpensive microelectromechanical system (MEMS) devices include miniature accelerometers, angular rate sensors, gyroscopes, etc., which may be combined to form a MEMS inertial measurement unit (IMU). MEMS gyroscopes offer low cost, compact size, low weight, and low power consumption, but are far less accurate than fiber-optic or ring-laser gyros. Unfortunately, when several MEMS gyroscopes, or angular rate sensors, are combined with several MEMS accelerometers, and other electronics, to form a MEMS IMU, the cost, weight, size and power consumption increase substantially over a single MEMS gyroscope.
Thus, a need remains for an inexpensive, small, lightweight, low power underwater navigation system that can estimate both latitude and longitude to the same accuracy, such as, for example, 10 nm.